At present, a wideband wavelength-tunable laser that can arbitrarily select a wavelength to be used and has a single light source has come into use in a WDM (Wavelength Division Multiplexing) optical communication system. The use of such a wideband wavelength-tunable laser can reduce the number of types of light sources. Therefore, the wideband wavelength-tunable laser can be manufactured in a small size and at a low cost. Recently, the WDM optical communication system using a ring topology has been developed to further improve the degree of freedom of the system. In this system, an arbitrary wavelength changing by a wavelength-tunable laser, like a RODAM (Reconfigurable Optical Add Drop Multiplexing) function, has come to be actively used.
However, in the case of using the wavelength-tunable laser in the WDM system, the laser has to satisfy two characteristics described below. First, high-precision wavelength control is required. The control precision of the laser oscillation wavelength has to satisfy the range of ±5% of the channel-spacing with respect to the standard-channel wavelength used in the system (hereafter, it is referred to as ITU-grid (International Telecommunication Union grid)). Second, it is required to change the laser wavelength without interference with the ITU-grid that is already used in the system when the laser wavelength is changed (it is called dark-tuning). In consideration of satisfying this characteristic, the use of a mode-hopping-free wavelength-tunable laser is the best, and the use of a wavelength-tunable laser capable of changing the oscillation wavelength into a desirable channel under the grid-hopping-free condition is the next best.
Wavelength-tunable laser modules having various structures are being developed to satisfy two characteristic-requirements described above. Patent Document 1 discloses a wavelength-tunable laser for tuning the laser oscillation wavelength into the ITU-grid by a filter having periodic transmission/reflection characteristics, thereby achieving high-precision wavelength control. An external cavity wavelength-tunable laser (ECTL) described in Patent Document 1 includes an etalon filter having a periodic transmission characteristic, a semiconductor optical amplifier (SOA) having phase control regions (PC regions) integrated therein, and a liquid-crystal wavelength-tunable mirror (LC mirror). The ETCL controls a peak reflection wavelength of the LC mirror and a phase of a resonator mode to be tuned into one of the periodic fixed peak transmission wavelengths of the etalon. The periodic peak wavelength of the etalon can be precisely tuned into the ITU-grid by adjusting the angle and temperature of the etalon.
However, there is a major problem in dark-tuning when a wavelength is changed in the wavelength-tunable laser described in Patent Document 1. It will be described below with reference to FIG. 6. FIG. 6 is a graph showing a time-variation comparison among an SOA current 601, a light output 602, an SOA active layer temperature 603, a phase shift 604 caused by a change in the SOA active layer temperature 603, and an oscillation wavelength 607 on the same time axis when the wavelength is changed.
As shown in FIG. 6, a wavelength-change is generally performed in (1) to (3) processes described below.
(1) A current applied to the SOA (the SOA current 601) is decreased down to 0 mA, and the light output 602 is turned off.
(2) A peak reflection wavelength is set at a desired wavelength by controlling a voltage of the LC mirror. At the same time, a current applied to the PC region (the PC current) is adjusted to a setting current (a current value whose mode is stabilized when the SOA current 601 becomes a desired driving value).(3) The SOA current 601 is increased up to a driving-condition value (200 mA, for example), and the light output 602 is turned on.
The SOA current 601 is greatly changed (0 mA->200 mA) in the (1) to (3) processes described above. When the SOA current 601 is greatly changed, a large amount of the phase shift 604 is caused. If a phase is shifted by a half cycle (π) from an optimum condition due to the phase shift 604, a mode-hopping is caused in the oscillation wavelength 607. The mode-hopping is frequently associated with a grid-hopping. Therefore, some measure needs to be taken to achieve the dark-tuning.
Here, as shown in FIG. 6, the phase shift 604 is caused due to heat generation by the SOA current 601. The heat generation is proportional to about the square of the SOA current 601, and the SOA active layer temperature 603 is changed by the heat generation. A refractive index of a semiconductor optical amplifier region (an SOA region) is changed due to a thermo-optic effect of semiconductor caused by the change of the SOA active layer temperature 603 (a positive change of the refractive index). The change of the refractive index causes a phase variation, or the phase shift 604. In comparing a current of 200 mA to be regularly used with the large amount of the SOA current 601, as shown in FIG. 6, a large phase variation of π or more is caused when the wavelength is changed. To solve the above-mentioned problem, Patent Document 2 proposes a method in which a wavelength changing value is extracted by a wavelength-monitoring structure prepared outside a laser, thereby controlling a wavelength by negatively feeding back the changing value.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-204195    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 7-111354